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The low symmetry martensite phase in Nickel-Titanium (NiTi) shape memory alloy (SMA) has a hierarchical microstructure with micro-scale crystallites, which themselves consist of nano-laminates with an internal twin structure. The martensite phase deforms inelastically though reorientation of the twin structure. Furthermore, structural features in the specimens such as cracks or notches interact with the microstructure, influencing the deformation of martensite. In this work, using multi-scale experiments and macro-scale modeling, we demonstrate that the deformation of martensite in NiTi at the macro scale in a notched specimen is largely determined by the structural features, and the hierarchical nature of the microstructure plays a smaller role. At the micro-scale, in-situ measurements indicate that the martensite microstructure continuously evolves during tensile loading. However, the nano-scale twin laminate structure persists despite the large imposed deformation and the presence of stress concentrating features such as a notch. This observation is counter to the intuition of martensite twins reorienting to large domains of the most favored crystallographic variant at larger loads. This persistence of a nano-laminate structure rather than coarsening into large domains of individual variants indicates that the deformation field around cracks/notches in martensite may be modeled for fracture-related phenomena with adequate accuracy using coarse-grained models, without a compelling need for martensite single-variant-scale models.

This study investigated the effect of stress-induced martensite aging under tensile and compressive stresses on the functional and viscoelastic properties in Ni50.3Ti32.2Hf17.5 polycrystals containing dispersed H-phase particles up to 70 nm in size obtained by preliminary austenite aging at 873 K for 3 h. It was found that stress-induced martensite aging at 428 K for 12 h results in the appearance of a two-way shape memory effect of −0.5% in compression and +1.8% in tension. Moreover, a significant change in viscoelastic properties can be observed: an increase in internal friction (by 25%) and a change in elastic modulus in tensile samples. The increase in internal friction during martensitic transformation after stress-induced martensite aging is associated with the oriented growth of thermal-induced martensite. After stress-induced martensite aging, the elastic modulus of martensite (EM) increased by 8 GPa, and the elastic modulus of austenite (EA) decreased by 8 GPa. It was shown that stress-induced martensite aging strongly affects the functional and viscoelastic properties of material and can be used to control them.

The effects of stress-free and stress-assisted aging at 673 K on microstructures, thermoelastic stress-induced B2-L10 martensitic transformation and superelasticity were investigated in [001]B2-oriented Co35Ni35Al30 (at. %) single crystals in compression. The aging of quenched crystals results in the formation of high-strength nanostructured composites containing spherical nanosized particles and elongated e-Co (HCP structure) particles, which do not undergo martensitic transformation. The stress-free and stress-assisted aged at 673 K for 0.5 h Co35Ni35Al30 crystals, oriented along [001]B2-direction, demonstrate high-yield stress levels for the B2-phase of 1360÷1630 MPa and a wide superelasticity temperature range from 193 K to 563 K with good cyclic stability and narrow stress hysteresis of Δσ = 50–60 MPa. It has been shown experimentally that a difference in the effective Young's modulus of austenite and martensite determinates a decrease in the reversible strain with a test temperature in both quenched and aged crystals. The tensile two-way shape memory effect with a reversible strain of eTWSME = +2.2% along the [001]B2-direction is induced due to the oriented growth of elongated e-Co-particles in stress-assisted aged crystals. These results provide a clear demonstration of the potential of aged CoNiAl single crystals for applications involving high stresses and temperatures.

B2–L21 configurational order resulting from thermal processing of Ni–Co–Mn–In Heusler alloys is one of the main factors defining the characteristics of the martensitic transformation experienced by these alloys. While the effects of the quenched-in disorder on the martensitic transformation have been extensively investigated, the kinetics of the ordering process is not well known and mostly inferred from the indirect results such as the dependence of the martensitic transformation or magnetic transition temperatures to thermal processing. However, recent results suggested that the kinetics of B2– L 2 1 ordering could have a more significant role than expected in the final state of the material and its functional properties. In the current work, an attempt has been made to investigate the kinetics of B2–L21 ordering in order to evaluate the dependence of the quenched-in disorder on the different variables of the thermal procedures. Results showed that ordering rate has a strong dependence on annealing temperature. Below 600 K, slow kinetics is forcing the crystal to always contain a certain amount of quenched-in disorder at room temperature. Between 600 K and 750 K, kinetics is fast enough to reach thermal equilibrium state within tens of minutes during the annealing process. Above 750 K, kinetics is very fast and thermal equilibrium can be reached during heating, so no isothermal holding is required. These predictions have been confirmed using differential scanning calorimetry and in-situ high-energy X-Ray diffraction measurements for non-direct estimations of the degree of order.

In the present study, a two-step thermo-mechanical processing consisting of cold work and heat treatment steps was performed to increase the operating temperature of 316 austenitic stainless steels. A hierarchical microstructure of thermally-stable, nano twin bands was achieved forming into bundles in elongated grains. The mechanical response of the samples with this microstructure was evaluated through uniaxial tension tests at temperatures ranging from 20 °C to 500 °C and compared with those from the fully annealed samples. The results demonstrate that such hierarchical microstructure leads to a significant increase in the elevated temperature yield strengths due to the presence of nano-twin boundaries and resulting decrease in dislocation mean free path and increase in dislocation storage capacity. In fact, the yield strength ratio of the twinned and annealed samples increases with increasing temperature up to 500 °C, indicating the effectiveness of pre-existing thermally-stable twin boundaries as the strengthening source at temperatures as high as 0.46 homologous temperature. The hierarchical microstructure also led to irregular serrations through dynamic strain aging in the stress-strain response at 500 °C, which is attributed to the bi-modal microstructural length-scales present in the structure affecting the diffusion distances during dynamic strain aging. This structure also increases the tensile strength, and without a total loss in ductility, even though the flow stress of the twinned samples surpasses the tensile strength of the annealed samples, especially at elevated temperatures. Total hardening rate is consistently higher in the twinned samples as compared to the annealed samples, indicating the positive role of nano-twin boundaries in the dislocation storage capacity at elevated temperatures. Overall, the present study clearly demonstrate the positive role of thermally stable nano-twins on the elevated temperature mechanical response of austenitic stainless steels.

The main disadvantage of fcc (face-centred cubic lattice) high-entropy alloys is the low stress level at the yield point (σ0.1) at a test temperature above room temperature. This restricts their practical application at high test temperatures from 773 K to 973 K. In this study, we found that a high stress level was reached at the yield point σ0.1 ≈ G/100–G/160 (G is the shear modulus) of the [001]- and [1¯44]-oriented crystals of the Co23.36Cr23.29Fe23.80Ni21.88Al7.67 (Al0.3CoCrFeNi) high-entropy alloy (HEA) within a wide temperature range of 77–973 K under tension, due to the occurrence, of nanotwins, multipoles, dislocations under plastic deformation at 77 K and the subsequent precipitation of ordered L12 and B2 particles. It was shown that grain boundaries are not formed and the samples remain in a single-crystal state after low-temperature deformation and subsequent ageing at 893 K for 50 h. Achieving a high-strength state in the Al0.3CoCrFeNi HEA single crystals induces the orientation dependence of the critical resolved shear stresses (τcr) at T ≥ 200 K (τcr[1¯44] > τcr[001]), which is absent in the initial single-phase crystals, weakens the temperature dependence of σ0.1 above 573 K, and reduces plasticity to 5–13% in the [1¯44] orientation and 15–20% in the [001] orientation.

The oxidation behaviours of polycrystalline and monocrystalline Ni-base alloys (Ni-16Cr-8Fe (wt%)) were investigated in situ by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The oxide layer formed is duplex: an inner chromium-rich layer and an outer layer which is rich in nickel and iron. Time evolution of the composition of inner layer from pure Cr2O3 towards spinel (NiCr2O4) was observed. The oxidation kinetics was accelerated by alloy grain boundaries, which can favour the diffusion of chromium. Both the parabolic and volatilization constants increased with increasing temperature, with kp varying from 1.9×10−3 to 1.6×10−2 nm2.s−1, and kv from 1.0×10−3 to 2.9×10−3 nm.s−1.

WOS: 000374615700008, The shape memory behavior of [111]-oriented Ni51Ti49 (at.%) single crystals was investigated after stress assisted aging at 500 degrees C for 1.5 h under a compressive stress of -150 MPa. It was found that a single family of Ni4Ti3 precipitates with two crystallographically equivalent variants was formed after aging under compressive stress. Stress-assisted aging resulted in tensile two-way shape memory effect strain of 1.56% under -5 MPa. Thermal cycling under -600 MPa resulted in a transformation strain of -2.15%, while the subsequent thermal cycling under -5 MPa resulted in a tensile two-way shape memory effect strain of 2.2%., RFBR Project [10-03-0154-a]; NASA [NNX11AQ31A], This work was supported in part by the NASA Fundamental Aeronautics Program, Supersonics Project and the NASA EPSCOR program (Grant No. NNX11AQ31A), and RFBR Project (Grant No. 10-03-0154-a).

The conventional shape memory effect (SME) and pseudoelasticity (PE) in as-grown [100] single crystals of Co49Ni21Ga30 alloy under compression are reported. The parent single crystals exhibit about 5% transformation strain at compressive stress levels as low as 4 MPa, and a pseudoelastic strain of 4.5%. Complete PE was observed in the temperature range from 35 to 285°C, along with increasing stress hysteresis with temperature. The latter is attributed to increasing number of variants and the corresponding variant–variant interactions. We demonstrate that the current material can be utilized in applications that demand high strength at elevated temperatures. Moreover, the current results also indicate the potential of this material to exhibit magnetic shape memory effect, which could broaden the scope of utility of this material upon further research.